Justice Secretary Michael
Gove is one of the high-profile Conservatives campaigning for a
Brexit.Dylan Martinez /
Reuters

The world of EU referendum opinion polls took a stunning turn on
Tuesday when ICM's most recent phone poll indicated a
four-point lead for Leave.

Remarkably, this meant that there had been a 14-point swing in
favour of a Brexit since ICM's previous phone poll which was
conducted just two weeks before.

The result also meant that the recent trend of surveys conducted over the
phone consistently showing comfortable leads for the Remain
campaign was broken.

It would be unwise for Leave campaigners to get overly
excited about the poll, though. That's because there are 2
important issues to consider when assessing its significance.

These issues were explained by poll analyst Matt Singh in his
latest post on the Number Cruncher Politics
blog.

Polls conducted following key events often produce overshoots

As Sign explains in his post, polls produced following major
events often produce dramatic but
short-lived results.

This was the case immediately after the draft version of
Britain's renegotiated EU membership was published — certain
polls showed huge leads for the Leave campaign, which didn't
last.

It is important to note that ICM's latest phone poll was
conducted shortly after the latest net migration figures were
released.

These figures boosted Leave's cause as they showed that David
Cameron was failing to fulfill past promises on reducing
immigration to Britain.

Of course, it is possible that public opinion is shifting
significantly in favour of Britain leaving the 28-nation bloc.
But this conclusion can only be confidently reached if this
trend sustains in polls published in the coming weeks.

Until that happens, the possibility remains that ICM's latest
poll was merely an overshoot born in the immediate aftermath of
the net migration figures.

Net
migration to Britain neared a record high in the year ending
December 2015.Getty

Experts are suspicious about polls conducted over bank holiday
weekends

The second reason why the ICM poll should be approached with
caution is that it was based on data that was gathered on a bank
holiday weekend.

Phone polls conducted on these weekends in the past have thrown
up really odd results. Singh believes that this might be because
an abnormally amount of people are away from their homes, meaning
samples can be skewed.

An example that he cited was when general election voting
intention polls conducted in December 2014 would randomly diverge at least partly because they
fell on Christmas holidays.

But the 'bank holiday weekend effect' theory requires more
evidence to be developed further. YouGov's Anthony Wells said the
impact it has on polling remains unclear in a post published on the UK Polling Report.

However, it is clear that the remarkable results of ICM's latest
poll can only really be understood when assessed alongside
surveys that will be published in the coming days and weeks.